Generally, signal processing devices are susceptible to transient surge signals transmitted from the transmission lines. In designing electrical sockets, it is very important to prevent the transient surge signals from transmitting to the signal processing devices via the electrical sockets because the signal processing devices are readily damaged by the transient surge signals. Furthermore, according to many safety regulations, surge tests should be performed before the electrical sockets come into the market.
Hereinafter, a schematic circuit diagram of a conventional electrical socket is illustrated with reference to FIG. 1.
In FIG. 1, the electrical socket 10 comprises a surge protection circuit 11 and a power supply circuit 12. The power supply circuit 12 of the electrical socket 10 has a socket structure (not shown) to be electrically connected with a signal processing device 13 such as any kind of electrical appliance or electronic device. Via the electrical socket 10, the electrical signals from the signal lines L, N and G are transmitted to the signal processing device 13. The operation principles of the power supply circuit 12 and the signal processing device 13 are well known in the art, and are not intended to redundantly describe herein. The principle of operating the surge protection circuit 11 will be illustrated as follows.
In order to prevent damage of the signal processing device 13 due to surge signals, surge absorbers are interconnected between any two signal lines of these three signal lines L, N and G. As shown in FIG. 1, the surge protection circuit 11 includes a Line-to-Neutral surge absorber 111, a Neutral-to-Ground surge absorber 112 and a Line-to-Ground surge absorber 113. Each of the surge absorbers 111, 112 and 113 is implemented by one, two or three metal oxide varistors (MOVs). Since the surge energy adsorbed by the metal oxide varistors is typically converted into heat, thermal fuses 114 and 115 are electrically connected to the input terminals of the signal lines L and G in series. In accordance with a protective measure, the electrical signal loop is instantly interrupted when the temperature surrounding the thermal fuse 114 and/or 115 reaches a predetermined value. In practical, for facilitating detecting the ambient temperature, the thermal fuse 114 is arranged in the vicinity of the surge absorber 111, and the thermal fuse 115 is arranged in the vicinities of the surge absorbers 112 and 113.
In an alternative approach, a break-switch 116 is connected to the input terminal of the fire line L. In a case that the current flowing through the fire line L is greater than a specific current rating (e.g. 50 amperes), the latch of the break-switch 116 is released for permitting interruption of the overcurrent. Since the process of releasing the latch is instantaneous, the resulting transient surge signals are difficult to be coped with. Therefore, the surge absorbers 111, 112 and 113 are indispensable for protecting the signal processing device 13.
Since the safety regulations become more severe, the surge protection circuit 11 fails to pass these safety regulations and deal with the surge problems. The reasons of causing these problems are illustrated as follows.
First of all, the surge protection circuit 11 and the power supply circuit 12 are mounted on a circuitry substrate 101 (as shown in FIG. 2). In addition, several metallic traces are formed on the surface of the circuitry substrate 101. According to the earlier UL (Underwriters Laboratories Inc.) safety regulations, the electrical connection between the metallic traces of the power signal lines and the surge absorber 111, 112 or 113 is interrupted if a overcurrent greater than 1,000 amperes is absorbed by either of the surge absorbers 111, 112 and 113. For example, as shown in FIG. 1, the trace t11 between the surge absorber 111 and the node N11 and the trace t12 between the node N12 and the thermal fuse 115 should be operated in the interrupted states if the tested overcurrents flow therethrough. Under this circumstance, the surge absorber in a short-circuit state when the overcurrent flows therethrough would no longer form a leakage current path to communicate with the signal processing device 13.
Unfortunately, at that moment when the overcurrents flow the traces t11 and t12, the traces t11 and t12 are readily subject to carbonization, which is unexpected or controlled. As a consequence, other electronic components or traces surrounding the traces t11 and t12 and on the circuitry substrate 101 are adversely affected to form another leakage current path.
Moreover, according to the safety regulations of UL 1449, second version, the equipment should be additionally capable of withstanding high current of 50˜150 amperes. As known, since the line widths of the traces t11 and t12 fail to be precisely controlled, the line widths are usually too narrow or broad. In a case that the line widths are too narrow, the surge absorbers 111, 112 and 113 are operated in the interrupted states before entering the breakage region to eliminate and absorb the surge energy. Under this circumstance, the surge signals are possibly fled to the signal processing device 13, and thus the signal processing device 13 is damaged. In a case that the line widths are too broad, the time period for allowing the current to flow through the surge absorbers 111, 112 and 113 is extended. In addition, these surge absorbers become very hot to cause ignition.
As a consequence, the surge protection circuit 11 fails to meet the requirements of the safety regulations of UL 1449, second version. Since the new standard for Safety for Transient Voltage Surge Suppression (TVSS) is more severe than that of UL 1449 (second version), the surge protection circuit 11 fails to withstand transient voltage surge 6 KV/3 KA.
In order to avoid generation of leakage current path due to carbonization of the traces t11 and t12, an isolating casing is employed. Please refer to FIG. 2, which is a layout structure illustrating a surge absorber and an isolating casing of a conventional electrical socket. Such layout structure is disclosed in Taiwanese Patent No. M273813, and the contents of which are hereby incorporated by reference. As shown in FIGS. 1 and 2, the surge absorber 111 and the thermal fuse 114 are mounted onto the circuitry substrate 101, and then covered with an isolating casing 117. Therefore, on account of convenience and normal operation of the surge absorber, broader line widths of the traces interconnected between the surge absorber 111 and the power signal lines of the circuitry substrate 101 are rendered. On the other hand, if the surge absorber 111 is suffered from ignition due to overheating, the surge absorber 111 is isolated from the surroundings by the isolating casing 117.
The layout structure of FIG. 2, however, increases the fabricating cost. In addition, since the line widths are widened, the time period for allowing the current to flow through the surge absorber 111 is extended, and thus the traces fail to be interrupted in a designated time interval. In other words, the layout structure of FIG. 2 is unable to withstand the high current test of 50˜150 amperes according to the safety regulations of UL 1449, second version and unable to withstand transient voltage surge of 6 KV/3 KA according to the standard for Safety for Transient Voltage Surge Suppression (TVSS).
For solving these problems, another electrical socket 30 is illustrated. The electrical socket 30 comprises a surge protection circuit 31 and a power supply circuit 32. The power supply circuit 32 of the electrical socket 30 has a socket structure (not shown) to be electrically connected with a signal processing device 33. Via the electrical socket 30, the electrical signals from the signal lines L, N and G are transmitted to the signal processing device 33. Most components included in the surge protection circuit 31 are similar to that shown in FIG. 1, and are not intended to redundantly describe herein. The surge protection circuit 31 is distinguished by the trace t31 between the surge absorber 311 and the node N31 and the trace t31 between the node N32 and the thermal fuse 315. Each of the traces t31 and t32 is coated with an isolating lacquer I, the problems resulted from carbonization are solved. This electrical socket, however, fails to pass the new standard for Safety for Transient Voltage Surge Suppression because the surge absorbers 311, 312 and 313 are still suffered from ignition due to overheating.
In views of the above-described disadvantages resulted from the prior art, the applicant keeps on carving unflaggingly to develop a surge protection circuit according to the present invention through wholehearted experience and research.